The present invention relates to the installation of taut reinforcements such as strand parts inside a casing to produce a stay belonging to the suspension system for a civil engineering construction.
In a stayed suspension, one or more towers support a structure, such as the deck of a bridge for example, via a collection of stays following oblique paths between a tower and the structure. A stay is a cable made up of a collection of reinforcements stretched between two end anchor points and generally surrounded by a casing. These reinforcements are often metal strand parts. In the case of a stayed bridge, each reinforcement is anchored to a tower and to the deck of the bridge that it plays a part in supporting.
European Patent 0 421 862 describes a method for tensioning the strand parts of a stay which advantageously allows the tensions to be evened out across the various strand parts while at the same time using a single strand part jack, which is far more lightweight and easier to handle (especially on a tower) than a communal jack. According to that method, a first strand part is tensioned to form a control strand part. Each subsequent strand part is tensioned using the single strand part jack until its tension matches that of the control strand part. In the course of this operation, the tension in the strand parts already anchored decreases a little as the tension in the new strand part increases. Gradually, this procedure ensures that the various strand parts in the stay are tensioned to the same value.
For large constructions, the stays used are typically very long, of a length which may be as much as several hundred meters, and a high number of taut elementary reinforcements (strand parts or the like) has to be provided in order to withstand the load.
Furthermore, on stayed constructions with a very long span (in excess of 500 meters), the drag load on the sheaf of stays is dominant over the action of the wind on the deck and may lead to appreciable over-engineering of the towers. As the drag is 10 proportional to the diameter of the casing, it is therefore desirable to provide stays with a small-diameter casing, that is to say stays that are more compact.
It is thus necessary to reach a delicate compromise between the number of strand parts per stay, that needs to be maximized in order to increase the supporting capacity of the stay, and its diameter, that needs to be minimized for aerodynamic reasons.
Now, it is generally necessary, within the casing, to provide enough space for the reinforcements to run in when the stay is being put in place. This is because the stays used in large bridges are very heavy, which means that it is not conceivable for them to be hauled up into position having prefabricated them on the deck or at a prefabrication area. In general, the casing is set in place along the oblique path of the stay, then the strand parts are installed one by one, or in small groups, hauling them up into position using a shuttle sliding along inside the casing and driven by a winch placed on the tower. When the last strand part (or the last group) is being hauled up, there still needs to be enough space in the casing to allow the shuttle to pass. It is clearly desirable for this remaining space to be minimized, in the search for the above compromise.
EP-A-O 654 562 gets around this problem by making the casing out of several shell sections assembled around the bundle of strand parts once the latter has been tensioned, thus allowing only a minimum amount of space to be left. However, for the overall design of the stay, it is decidedly preferable to provide a casing made as a single piece rather than made in several parts. This in particular affords the reinforcements better protection against environmental attack.
One object of the present invention is to provide a satisfactory solution to the abovementioned problem.